This invention relates to a nuclear magnetic resonance (NMR) imaging method and more particularly to a method for absorbing time change of the static magnetic field in an NMR imaging device to remove the positional deviation of an NMR image on measurement thereof.
An NMR imaging (simply referred to as MRI) is a system for imaging anatomic or biochemical information based on the protons or other nuclides. The image therein includes a nuclear spin density image itself, the nuclear spin density image enhanced with a longitudinal relaxation time T.sub.1 or lateral relaxation time T.sub.2, and T.sub.1 calculated image or T.sub.2 calculated image. The NMR imaging makes it possible to see, as images, the parameters related with tissues, which provides medically important information in judging the abnormality of the tissue.
In measurement of NMR phenomena, an object to be examined is placed in a static magnetic field, which can be generated by a permanent magnet, resistive magnet or superconductive magnet, and is irradiated with RF (radio frequency) pulses (or selectively excited pulses). Subsequently, the resonance signals emitted from the object are received. The NMR imaging is carried out in such a way that the NMR signals are measured under the condition where a gradient magnetic field is applied and thereafter the resultant images are reconstructed. More specifically, the object is irradiated with the RF pulses under the condition where the gradient magnetic field is applied and the NMR signals produced from the examination region of the object are encoded as spatial information.
The gradient magnetic field is employed for encoding the space for the reason why a nuclear resonance frequency .omega. is in a linear relation with the magnetic field. Specifically, if the gradient magnetic field remains spatially linear, the spatial position of the examination region is linearly related with the resonance frequency. Thus, the position information of the object can be obtained by only Fourier transforming the NMR signals, which are time information, so as to be changed into a frequency axis. This can be employed to reconstruct the image.
However, if the static magnetic field is changed, a problem of position deviation occurs. The problem of the position deviation can be considered individually in a two-dimentional image and in a slice direction perpendicular thereto.
Generally, the NMR imaging device for medical use takes several minutes or more for normal measurement and thus uses a multi-slice technique to provide a number of images through one measurement. A number of images can be obtained by irradiating object slices with selectively excited pulses at the center frequency corresponding to the object slice in a condition where the gradient magnetic field is applied. Then, if the static magnetic field is being varied, the signal at different slice position from the predetermined slice position will be measured. This results in undesired anatomic position deviations.
The details of the above phenomenon will be explained with reference to FIG. 1. It is assumed that the center frequency corresponding to a prescribed slice position is .omega..sub.0, that is, the center frequency decided by the static magnetic field at the isocenter is .omega..sub.0. Now, it is assumed that the above center frequency is varied to .omega..sub.1 by the change of the static magnetic field due to the ambient temperature. In such a situation, when the slice position has been set so as to correspond to the center frequency of .omega..sub.0, as seen from the upper frequency axis of FIG. 1, the slice position is S.sub.0. However, if the center frequency has been varied to .omega..sub.1 due to the ambient temperature change, the center frequency .omega..sub.1 at the isocenter becomes a central point of the gradient magnetic field. If .omega..sub.1 &gt;.omega..sub.0, the slice position corresponding to the center frequency .omega..sub.0 is S.sub.1 which is a position leftwardly shifted from S.sub.0 in FIG. 1.
Also in the two-dimensional image, if the frequency decided by the static magnetic field at the isocenter in a static magnetic space is different from assumed center frequency, position deviation occurs in the direction of frequency encoding.
The above phenomenon will be explained in detail with reference to FIGS. 2 and 3. The detection of NMR signals is defined as taking a difference signal between the frequency of a received signal and a predetermined center frequency. It is now assumed that the predetermined center frequency .omega..sub.1 is different from the proton resonance frequency .omega..sub.0 decided by the static magnetic field at the isocenter in an imaging space.
FIG. 2 schematically shows a time sequence in a typical spin echo measurement. In FIG. 2, RF shows timings of irradiating RF signals and its envelope for selective excitation, G.sub.z shows timings of applying a gradient magnetic field in the slice direction; G.sub.y shows timings of applying a gradient magnetic field in the direction of phase encoding and the amplitudes thereof varied; G.sub.x showsw timings of applying a gradient magnetic field in the direction of frequency encoding. Signal shows NMR signals to be measured; and the lowest stage shows plural sections of the time-sequence.
During the section 1, the 90 degree selective excitation pulse is irradiated onto an object and also the slice direction gradient magnetic field is applied. During the section 2, the gradient magnetic field in the phase encode direction is applied to provide the rotation of a nuclear spin dependent upon the position in the y direction. During the section 3, any signal is not applied. During the section 4, the 180 degree selective excitation pulse is irradiated and also the gradient magnetic field in the slice direction is applied. During the section 5, the gradient magnetic field in the frequency encode direction is applied and the NMR signal is measured.
Meanwhile, although during the sections 1 and 4, the pulse at the frequency .omega..sub.z is irradiated, during the section 5 when the NMR signal is measured, the frequency must be essentially returned to the proton resonance frequency which is decided by the static magnetic field intensity at the isocenter in an imaging space. At this stage, if the predetermined center frequency .omega..sub.1 which is to be essentially equal to .omega.is different from .omega..sub.0, the position deviation will be measured in the direction of frequency encoding. More specifically, as already mentioned, the NMR signal is measured as the difference frequency from the center frequency so that the center of the resultant image corresponds to the frequency .omega..sub.0. Thus, if the predetermined frequency is not .omega..sub.0 but .omega..sub.1, the position deviation corresponding to the difference frequency .vertline..omega..sub.0 -.omega..sub.1 .vertline. will occur in the frequency encode direction on the two dimentional image (see FIG. 3). The direction of deviation depends upon if the frequency encode gradient magnetic field is positive or negative in the x-axis direction.
From the above explanation, it is apparent that change in the static magnetic field intensity gives rise to position deviation measured in imaging. To obviate this disadvantage, the technique of "magnetic field locking" which is disclosed in Kazuo Toori et al "JITSUYO NMR, CW.FT NMR NO TSUKAIKATA" Aug. 10, 1985, pp. 67-69, has been commonly used. This magnetic field locking is a technique in which when the static magnetic field is changed from the assumed central magnetic field at the center of imaging, it is increased or decreased by the difference to coincide with the predetermined center magnetic field, that is, to lock it. To implement this technique, the NMR imaging device using a permanent magnet or resistive magnet is provided with shim coils which are capable of changing the static magnetic field; the assumed center magnetic field is so controlled as to be constant by coil current.
However, this magnetic field locking technique has the following disadvantages. If the magnet having a negative temperature coefficient is used as in the permanent magnet NMR imaging device, the current flowing through the shim or correcting coil generates heat, thus further decreasing the static magnetic field, which requires the current to be further increased. Such a vicious circle can be cut by slightly cancelling the static magnetic field generated in the permanent magnet by always passing the current through the shim coil. Specifically, if the current to be passed through the coil is decreased when the ambient temperature is increased and the static magnetic field generated by the permanent magnet is decreased, the static magnetic field can be restored to the assumed magnetic field at the center of imaging and also the current generation is decreased due to the decreased current.
Such an arrangement, however, requires that the magnetic field to be cancelled by the shim coil is supplemented by intensifying the permanent magnet. This disadvantageously increases the fabrication cost of the static magnetic field generating device using permanent magnet as an NMR imaging device.